I. Field
The present invention relates generally to data communication, and more specifically to techniques for predicting weights used for closed-loop transmit diversity in wireless communication systems.
II. Background
In a wireless communication system, data to be transmitted is first modulated onto a radio frequency (RF) carrier signal to generate an RF modulated signal that is more suitable for transmission over a wireless channel. The transmitted RF modulated signal may reach a receiver via a number of propagation paths. The characteristics of the propagation paths may vary over time due to various factors such as, for example, fading and multipath. Consequently the transmitted RF modulated signal may experience different channel conditions and may be associated with different complex channel gains over time.
To provide diversity against deleterious path effects and improve reliability, multiple transmit antennas and/or multiple receive antennas may be used for data transmission. Transmit diversity is achieved by the use of multiple antennas to transmit data, and receive diversity is achieved by the use of multiple antennas to receive a data transmission. A transmission channel is formed between each of the transmit antenna(s) and each of the receive antenna(s). If the transmission channels for different transmit/receive antenna pairs are linearly independent (i.e., a transmission on one channel is not formed as a linear combination of the transmissions on the other channels), which is generally true to at least an extent, then diversity increases and the likelihood of correctly receiving a data transmission improves as the number of antennas increases.
For costs and other considerations, some wireless communication systems employ multiple antennas at a base station and a single antenna at a terminal for data transmission. On the downlink, transmit diversity may be achieved by transmitting data redundantly on multiple RF modulated signals from the multiple base station antennas redundantly on multiple RF modulated signals from the multiple base station antennas to the single terminal antenna. These signals typically experience different channel conditions and may be associated with different channel gains. Consequently, these signals typically arrive at the terminal antenna with different phases and amplitudes, and may add constructively or destructively.
A control loop may be maintained to determine weights to be applied to the multiple RF modulated signals, at the base station, such that these signals maximally combine at the terminal. The control loop would estimate the complex channel gain (which is also referred to as fading coefficient) between each of the multiple antennas at the base station and the single antenna at the terminal. The control loop would then determine the “optimal” weights for the RF modulated signals based on the estimated channel gains for the multiple base station antennas. The weights are then applied to the RF modulated signals prior to transmission from the base station antennas. By adjusting the phase and possibly the amplitude of the transmitted RF modulated signals, the received signals at the terminal can be assured to add constructively, and improved performance may then be achieved.
The performance of a closed-loop transmit diversity scheme, such as the one described above, is dependent on the optimality of the weights at the time that they are applied. Unfortunately, any closed-loop transmit diversity scheme will exhibit some amounts of delay between the time that the weights are computed to the time that they are applied. If the channel condition is not static or stationary during this entire delay (e.g., due to movement by the terminal), then the weights that may have been optimal at the time that they are computed may be far from optimal at the time that they are applied. This would then degrade performance.
There is therefore a need in the art for techniques for predicting weights used for closed-loop transmit diversity in wireless communication systems.